For example, in a conventional color image encoding device described in the following nonpatent reference 1, an inputted color image is partitioned into largest coding blocks each having a predetermined size, and each largest coding block is further partitioned hierarchically into smaller coding blocks.
Each coding block is further partitioned into smaller prediction blocks, and an intra-screen prediction and a motion-compensated prediction are performed on each of the prediction blocks to generate a prediction error.
Further, the prediction error is divided hierarchically into transformation blocks within each coding block and each of the transform coefficients is entropy-encoded, thereby achieving a high compression ratio.
In a conventional color image encoding device, when encoding a YUV4:2:0 signal, by making block partitioning information about coding blocks, prediction blocks, and transformation blocks of the chrominance signals be the same as that of the luminance signal, the coding efficiency of the chrominance signals is improved without the necessity to encode information associated with the transformation block size of the chrominance signals.
At that time, except in a case in which a block to be partitioned of a chrominance signal cannot be partitioned because its size is a minimum of possible block sizes, the block size of the chrominance signal is one half of the block size of the luminance signal both in the vertical and horizontal directions.